In the past several decades contact lenses, and in particular soft hydrophilic contact lenses, have won wide acceptance among those requiring vision correction. The reason for this acceptance has been superior visual acuity, freedom from aberrant optical effects associated with spectacles (lack of peripheral vision, fogging, shifting of the lens) and improved personal appearance of the wearer.
It is well known that as an individual ages, the eye is less able to accommodate, i.e., bend the natural lens in the eye in order to focus on objects that are relatively near to the observer. This condition is referred to as presbyopia and presbyopes have in the past relied upon spectacles or other lenses having a number of different zones with different optical powers to which the wearer can shift his vision in order to find the appropriate optical power for the object or objects upon which the observer wishes to focus.
With spectacles this process involves shifting one's field of vision from typically an upper, far power to a different, near power. With contact lenses however this approach has been less than satisfactory. The contact lens, working in conjunction with the natural lens, forms an image on the retina of the eye by focusing light incident on each part of the cornea from different field angles onto each part of the retina in order to form the image. For instance, as the pupil contracts in response to brighter light, the image on the retina does not shrink but rather light coming through a smaller portion of the lens is used to construct the entire image.
It is known in the art that under certain circumstances that separate competing images can be discriminated by the brain accepting the in-focus image and rejecting the out of focus image.
One example of this type of contact lens used for the correction of the presbyopic by providing simultaneous near and far vision is described in U.S. Pat. No. 4,923,296 to Erickson. Described therein is a lens system which comprises a pair of contact lenses having one eye with a near upper portion and a distant lower portion while the other eye contains a distant upper portion and near lower portion. Together these are said to provide at least partial clear images in both eyes, and through suppression by the brain of the blurred images, allows alignment of the clear image to produce an in-focus image. This system however requires a ballasting by peripheral, prism or weight to ensure the proper orientation of the lens on the eyes to achieve the above described affect.
Another attempt at providing a bifocal contact lens is described in European Patent Specification Publication No. 0107444; Application No. 83306172.4. Unlike the previous patent, the lens of this European Application does not require that the lens be oriented. The lens described in this application, however, is constructed by the use of different materials having different refractive indicia to achieve different optical powers or by having a different vision zones formed as a profile on the back surface of the lens. In addition, this lens could provide different ratios of near to far vision surface areas and may provide an insufficient amount of light for either the near or far field when the pupil passes through different diameters.
Prior art lenses using zones of a different refractive focal lengths were typically theoretical designs and not manufactured. This failure to realize an actual product is due to the inability to manufacture the type of lenses conceived. The production of contact lenses as well as intraocular lenses is performed by spin casting or precision lathe cutting. These processes produce radially symmetric lenses upon which it is extremely difficult to effect areas having different focal lengths because machining different curvatures around the lens is impossible.
One attempt known in the art to provide a method of compensating for presbyopia without complex lens manufacture is known as "monovision". In the monovision system a patient is fitted with one contact lens for distant vision in one eye and a second contact lens for near vision in the other eye. Although it has been found that with monovision a patient can acceptably distinguish both distance and near objects, there is a substantial loss of depth perception.
For these reasons although simple systems such as monovision are somewhat understood, more complex schemes for multifocal refractive lenses are primarily theoretical.
Another approach to producing a multifocal corrective eye lens involves the use of diffractive optics. One of the shortcomings of this approach, as with previously described types of multifocal lenses using radially symmetric, concentric near and far distance zones has been a deficiency in near vision, particularly at low light levels. In a diffractive design only about 40% of the light incident on the lens is used for near vision with another 40% being used for far vision. The remaining 20% is not used for either near or far vision, but rather is lost to higher orders of diffraction and scatter effect. This represents the best theoretical case and in manufacturing reality even less light is available due to manufacturing difficulties. Difficulty of manufacture in general represents another shortcoming of diffractive lenses since the diffractive surface must be to tolerances on the order of the wavelength of light.
It is an object of the present invention to provide a bifocal contact lens which is not sensitive to orientation and therefore does not require any type of ballasting or weighting but provides adequate depth perception.
It is further object of the invention to provide a lens that has a controlled ratio of areas for different focal lengths regardless of pupil diameter.
It is another object of the invention to provide a multifocal refractive lens for focusing light on the eye which contains at least one optical power having a surface curvature which is aspheric and provides a smooth boundary with its adjacent segments.
It is a further object of the invention to provide a method of producing multifocal lenses using lens surface molds to provide the multifocal optical powers. The lens surface molds are separated into interchangeable segments which can be assembled to provide a segmented multifocal lens which is then used to mold a lens using the lens mold surface.